1. Field of the Invention
The present invention pertains to a multi-color image forming apparatus and method that express darkness levels by changing the ratio of dots in each pixel, based on pixel gradation signals, to form a multi-color image.
2. Description of the Prior Art
As a consequence of the popularity of personal computers for home use, image forming devices such as full color printers have come to be increasingly used in households as well as by businesses.
Dithering is one method by which to express gradation using an image forming device employing the electrophotographic method.
Specifically, dithering involves contrasting and comparing input image information 70 of the input image shown in FIG. 6(c) and each threshold component of the threshold matrix shown in FIG. 6(a), and expressing gradation by means of control such that if input image signal of each pixel is larger than the corresponding threshold component, as shown in FIG. 6(c), dots are formed at positions corresponding to the threshold components, while if input image signal of each pixel is smaller than the corresponding threshold component, dots are not formed. Here, in FIG. 6(c), the input image level is assumed to be 7. In addition, a 16-level image reproduction is possible using the threshold matrix of FIG. 6(a), but the gradation level may be increased through duty modulation controlling the time of laser irradiation, as shown in FIG. 6(b).
Besides the dithering described above, a method is also used in which, as shown in FIGS. 7(a) through 7(h), output patterns corresponding to each level of input image information are set beforehand, a look-up table (LUT) is prepared, and an image is output with reference to the prepared look-up table.
When applying these gradation expression methods to the formation of color images, for example, as shown in FIGS. 9(a) and 9(b), where a black image and a magenta image are placed one over the other using the same threshold matrix, due to a lack of uniformity of distance between the output dots or misalignment of the printed characters resulting from uneven pitch, etc., caused by uneven rotation of the photoreceptor, etc., the dots may be placed directly on top of each other, as shown in FIG. 9(c), or they may conversely be placed in alternating locations, as shown in FIG. 9(d). When this occurs, if the dots are placed directly on top of each other as shown in FIG. 9(c), the color appears closer to black, whereas if the dots are placed in alternating positions as shown in FIG. 9(d), the color appears closer to red, such that if an image of uniform density is printed, unevenness in color occurs due to the discrepancy in the way the dots are placed one on top of the other.
Therefore, it has been considered how to prevent the occurrence of color unevenness by establishing a screen angle for each gradation pattern image corresponding to each color in order to disperse the placement of the dots on top of each other.
For example, in dithering, the screen angle may be set by arranging a plurality of unit gradation patterns produced by unit threshold matrices like the one shown in FIG. 8(a). The unit gradation patterns are arranged in a non-sequential order. The gradation pattern image shown in FIG. 8(b) comprises unit gradation patterns like the one shown in FIG. 8(a), which are displaced horizontally to the right by distance `a`, or four pixels, and displaced vertically downwardly by distance `b`, or one pixel, therefore resulting in a screen angle .theta.1, corresponding to the direction of line c. The tan .theta.1=1/4 and screen angle .theta.1.apprxeq.14 degrees. It can also be said that the unit gradation patterns arranged as shown in FIG. 8(b) also comprises unit gradation patterns like the one shown in FIG. 8(a), which are displaced horizontally to the left by a distance b' of one pixel and displaced vertically downwardly by a distance a' of four pixels, therefore resulting in a screen angle .theta.2 corresponding to the direction of the line c', so that the screen angle .theta.2.apprxeq.104 degrees. In other words, the gradation pattern image by arranging unit gradation patterns as shown in FIG. 8(b) has two screen angles .theta.1 and .theta.2 that are perpendicular to each other. In other words, the screen angle .theta.2 equals screen angle .theta.1 plus 90.degree. and line c' is perpendicular to line c. This characteristic is to be called the two-directional characteristic of screen angles.
The resolution (number of lines) of the image that is output using the threshold matrix shown in FIG. 8(b) is inversely proportional to the length of the line c that is obtained by connecting the centers of two neighboring unit threshold matrices and is set such that the image resolution will be essentially the same for all the colors. Resolution will be indicated hereinafter as `c`. If the difference between the screen angles of the gradation pattern images for any two colors is small, the texture (rosettes) will stand out, as shown in FIGS. 10 and 11. Therefore, it is necessary that the difference between screen angles for any two colors be set as large as possible.
In the multi-color image forming apparatus and method using conventional dithering, however, because the number of threshold components that comprises the unit threshold matrix is limited to a certain number or less, only a limited number of discrete values are available for setting of the screen angles, and if it is attempted to give the image high resolution, the number of threshold components becomes further limited, as do the values of the screen angles that can be set. In addition, as described above, in conventional dithering, the gradation pattern image has two screen angles .theta.1 and .theta.2 that are perpendicular to each other, as a result of which the screen angles for gradation pattern images for the recording colors that can be set will be further limited.
As described above, in the conventional multi-color image forming apparatus and method, even if different screen angles are set for each gradation pattern image for each recording color, the difference between screen angles cannot be set to be large. Consequently, the problems arise that the occurrence of color unevenness cannot be adequately prevented or that ring-like texture (rosettes) will occur cyclically, reducing image quality.